As generally accepted in the art, there are currently five known glycoprotein hormone polypeptides produced in humans: alpha-subunit, TSH-(thyroid stimulating hormone)-β-subunit, FSH-(follicle stimulating hormone)-β-subunit, LH-(luteinizing hormone)-β-subunit, and CG-(chorionic gonadotropin)-β-subunit; Thotakura and Blithe, Glycobiology, Volume 5, pages 3-10 (1995); Wondisford et al. in Volume 1, Endocrinology (edited by L. DeGroot), pages 208-217, W. B. Saunders Company, Philadelphia, Pa. (1995); Moyle and Campbell, in Volume 1, Endocrinology (edited by L. DeGroot), pages 230-241, W. B. Saunders Company, Philadelphia, Pa. (1995). These polypeptides are produced by single genes, with the exception of the CG-β-subunit which is encoded by a multigene cluster composed of six homologous sequences linked to the single LH-β-subunit gene on chromosome 19; Bo and Boime, Journal of Biological Chemistry, vol. 267, pp. 3179-3184 (1992).
Monomeric alpha-subunit (FAS, or free alpha-subunit) has hormonal activity and is secreted by the pituitary gland and the placenta. FAS has been found to play a role in the differentiation of prolactin producing cells in the pituitary and placenta; see Begeot et al., Science, vol. 226, pp. 566-568 (1984), Van-Bael and Denef, Journal of Neuroendocrinology, vol. 8, pp. 99-102 (1996), and Moy et al., Endocrinology, vol. 137, pp. 1332-1339 (1996); and also to stimulate placental prolactin secretion; see Blithe et al., Endocrinology, vol. 129, pp. 2257-2259 (1991).
Alpha-subunit also heterodimerizes with each of the four beta-subunits to form four heterodimeric hormones (TSH, FSH, LH and CG). TSH, FSH and LH are produced in the pituitary, stored in secretion granules, and secreted when the appropriate releasing hormone is produced by the hypothalamus. CG is produced in the placenta and appears to be secreted constitutively (it is not stored in secretion granules); see Wondisford et al. in Volume 1, Endocrinology (ed. L. DeGroot), pp. 208-217, above, and Hall and Crowley, Jr. in Volume 1, Endocrinology (ed. L. DeGroot), pp. 242-258, W. B. Saunders Company, Philadelphia, Pa. (1995).
TSH influences basal metabolism by regulating the production of thyroid hormones and is used clinically for enhancing the detection and treatment of thyroid carcinoma; see McEvoy, G. (ed.), AHFS Drug Information, pp. 2041-2042, American Society of Health-System Pharmacists, Inc., Bethesda, Md. (1998). In addition, diagnostic tests for measuring TSH levels in the blood are commonly used for determining the functional status of the thyroid gland when thyroid gland disorder is suspected.
FSH and LH play important roles in the maintenance of reproductive function in males and females (i.e., gonadal maturation and gonadal steroid production). CG is involved in the maintenance of pregnancy by stimulating the corpus luteum to produce steroid hormones during the first trimester. FSH, LH and CG are used clinically to treat infertility and also as reagents in assisted reproduction procedures such as in vitro fertilization (IVF); see McEvoy, G. (ed.), AHFS Drug Information, pp. 2564-2567, American Society of Health-System Pharmacists, Inc., Bethesda, Md. (1998). Diagnostic tests for measuring FSH, LH and CG levels are used for the diagnosis of fertility disorders, as well as to test for pregnancy.
Naturally occurring metabolites of the above mentioned glycoprotein hormone polypeptides have been described, such as the β-core fragment which is derived from the beta subunit of CG, but no function has yet been assigned to these metabolites; Moyle and Campbell in Volume 1 Endocrinology (ed. L. DeGroot) pp. 230-241, above.
In 1994, the five known glycoprotein hormone polypeptides were placed into the cystine-knot growth factor structural superfamily, based on the crystal structure of human CG; Lapthorn et al., Nature, vol. 369, pp. 455-61 (1994). This superfamily includes the TGF-β (transforming growth factor beta), NGF (nerve growth factor) and PDGF (platelet-derived growth factor) gene families. The cystine-knot is formed by three intramolecular disulfide bonds, has a very characteristic structure, and is responsible for the overall three-dimensional structure of all of the members of the superfamily; Isaacs, Current Opinion in Structural Biology, vol. 5, pp. 391-395 (1995). A recently published patent application describes a novel member of the cystine-knot family (zsig51); Sheppard and Lok, (1999) WIPO patent application WO99/41377. zsig51 has in fact been determined to be a new, alpha-like, member of the glycoprotein hormone family and will thus be referred to here as “α2” or “alpha-2” [Paszty et al. (2000) WIPO patent application WO 00/78964].